Strain gauge and sensor module

ABSTRACT

A strain gauge includes a flexible substrate; a functional layer formed of a metal, an alloy, or a metal compound, on one surface of the substrate; a resistor formed of a Cr composite film, on one surface of the functional layer; and an insulating resin layer with which the resistor is coated.

TECHNICAL FIELD

The present invention relates to a strain gauge and a sensor module.

BACKGROUND ART

A strain gauge is known to be attached to a measured object to detectstrain of the measured object. The strain gauge includes a resistor thatdetects strain, and as resistor material, for example, materialincluding Cr (chromium) or Ni (nickel) is used. The resistor is formedon a substrate made of, for example, an insulating resin (see, forexample, Patent document 1).

CITATION LIST Patent Document

[Patent document 1] Japanese Unexamined Patent Application PublicationNo. 2016-74934

SUMMARY

Strain gauges have been required to be highly resistive in recent years,and material or the like of the resistor is considered.

In view of the point described above, an object of the present inventionis to render a resistor highly resistive with respect to a strain gaugeincluding the resistor formed above a flexible substrate.

A strain gauge includes a flexible substrate; a functional layer formedof a metal, an alloy, or a metal compound, on one surface of thesubstrate; a resistor formed of a Cr composite film, on one surface ofthe functional layer; and an insulating resin layer with which theresistor is coated.

Effects of the Invention

According to the disclosed technique, a resistor can be highly resistivewith respect to a strain gauge including the resistor formed above aflexible substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of an example of a strain gauge according to afirst embodiment;

FIG. 2 is a cross-sectional view of an example of the strain gaugeaccording to the first embodiment;

FIG. 3A is a diagram (part 1) illustrating an example of a process ofmanufacturing the strain gauge according to the first embodiment;

FIG. 3B is a diagram (part 2) illustrating an example of the process ofmanufacturing the strain gauge according to the first embodiment;

FIG. 3C is a diagram (part 3) illustrating an example of the process ofmanufacturing the strain gauge according to the first embodiment;

FIG. 4 is a cross-sectional view of an example of a strain gaugeaccording to modification 1 of the first embodiment;

FIG. 5 is a plan view of an example of a strain gauge according to asecond embodiment;

FIG. 6 is a cross-sectional view of an example of the strain gaugeaccording to the second embodiment;

FIG. 7A is a diagram (part 1) illustrating an example of a process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 7B is a diagram (part 2) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 7C is a diagram (part 3) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 7D is a diagram (part 4) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 8A is a diagram (part 5) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 8B is a diagram (part 6) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 8C is a diagram (part 7) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 8D is a diagram (part 8) illustrating an example of the process ofmanufacturing the strain gauge according to the second embodiment;

FIG. 9 is a cross-sectional view of an example of a strain gaugeaccording to modification 1 of the second embodiment;

FIG. 10 is a cross-sectional view of an example of a strain gaugeaccording to modification 2 of the second embodiment;

FIG. 11 is a cross-sectional view of an example of a sensor moduleaccording to a third embodiment;

FIG. 12 is a diagram illustrating a result of X-ray fluorescent analysisfor a functional layer;

FIG. 13 is a diagram illustrating a result of X-ray diffraction for aresistor;

FIG. 14 is a diagram illustrating a relationship between an expansioncoefficient of a substrate and internal stress of a resistor; and

FIG. 15 is a diagram illustrating a relationship between surfaceunevenness of a substrate and the number of pinholes of a resistor.

DESCRIPTION OF EMBODIMENTS

One or more embodiments will be hereinafter described with reference tothe drawings. In each figure, the same numerals denote the samecomponents; accordingly, duplicative explanations may be omitted.

First Embodiment

FIG. 1 is a plan view of an example of a strain gauge according to afirst embodiment. FIG. 2 is a cross-sectional view of an example of thestrain gauge according to the first embodiment, and illustrates a crosssection taken along the A-A line in FIG. 1 . With reference to FIGS. 1and 2 , the strain gauge 1 includes a substrate 10, a functional layer20, a resistor 30, terminal sections 41, and a cover layer 60. Note thatin FIG. 1 , an outer edge of the cover layer 60 is only expressed by adashed line in order to indicate the resistor 30, for the sake ofconvenience.

Note that in the present embodiment, for the sake of convenience, withrespect to the strain gauge 1, the side of the substrate 10 where theresistor 30 is provided is referred to as an upper side or one side; andthe side of the substrate 10 where the resistor 30 is not provided isreferred to as a lower side or another side. Further, for eachcomponent, the surface on the side where the resistor 30 is provided isreferred to as one surface or an upper surface; and the surface on theside where the resistor 30 is not provided is referred to as anothersurface or a lower surface. However, the strain gauge 1 can be used in astate of being upside down, or be disposed at any angle. Further, a planview means that an object is viewed from a direction normal to an uppersurface 10 a of the substrate 10, and a planar shape refers to a shapeof an object when viewed from the direction normal to the upper surface10 a of the substrate 10.

The substrate 10 is a member that is a base layer for forming theresistor 30 or the like and is flexible. The thickness of the substrate10 is not particularly restricted, and can be appropriately selected forany purpose. For example, such a thickness can be approximately between5 μm and 500 μm. In particular, when the thickness of the substrate 10is between 5 μm and 200 μm, it is preferable in terms of strain transferfrom a flexure element surface that is bonded to a lower surface of thesubstrate 10 via an adhesive layer or the like; and dimensionalstability with respect to environment, and when the thickness is 10 μmor more, it is further preferable in terms of insulation.

The substrate 10 can be formed of an insulating resin film such as a PI(polyimide) resin, an epoxy resin, a PEEK (polyether ether ketone)resin, a PEN (polyethylene naphthalate) resin, a PET (polyethyleneterephthalate) resin, a PPS (polyphenylene sulfide) resin, or apolyolefin resin. Note that the film refers to a flexible member havinga thickness of about 500 μm or less.

Here, the “formed of an insulating resin film” is not intended topreclude the substrate 10 from containing fillers, impurities, or thelike in the insulating resin film. The substrate 10 may be formed of,for example, an insulating resin film containing fillers such as silicaor alumina.

The functional layer 20 is formed, as a lower layer of the resistor 30,on the upper surface 10 a of the substrate 10. In other words, a planarshape of the functional layer 20 is approximately the same as the planarshape of the resistor 30 illustrated in FIG. 1 . The thickness of thefunctional layer 20 can be approximately between 1 nm and 100 nm, forexample.

In the present application, the functional layer refers to a layer thathas a function of promoting crystal growth of the resistor 30 that is atleast an upper layer. The functional layer 20 preferably further has afunction of preventing oxidation of the resistor 30 caused by oxygen andmoisture included in the substrate 10, as well as a function ofimproving adhesion between the substrate 10 and the resistor 30. Thefunctional layer 20 may further have other functions.

The insulating resin film that constitutes the substrate 10 containsoxygen and moisture. In this regard, particularly when the resistor 30includes Cr (chromium), it is effective for the functional layer 20 tohave a function of preventing oxidation of the resistor 30, because Crforms an autoxidized film.

The material of the functional layer 20 is not particularly restrictedas long as it is material having a function of promoting crystal growthof the resistor 30 that is at least an upper layer. Such material can beappropriately selected for any purpose, and includes one or more typesof metals selected from the group consisting of, for example, Cr(chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni(nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C(carbon), Zn (zinc), Cu (copper), Bi (bismuth), Fe (iron), Mo(molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium),Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver),Au (gold), Co (cobalt), Mn (manganese), and Al (aluminum); an alloy ofany metals from among the group; or a compound of any metal from amongthe group.

Examples of the above alloy include FeCr, TiAl, FeNi, NiCr, CrCu, andthe like. Examples of the above compound include TiN, TaN, Si₃N₄, TiO₂,Ta₂O₅, SiO₂, and the like.

The resistor 30 is a thin film formed in a predetermined pattern andabove the upper surface of the functional layer 20, and is a sensitivesection where resistance varies according to strain. Note that in FIG. 1, for the sake of convenience, the resistor 30 is illustrated in a crepepattern.

The resistor 30 can be formed of, for example, material including Cr(chromium); material including Ni (nickel); or material including bothof Cr and Ni. In other words, the resistor 30 can be formed of materialincluding at least one from among Cr and Ni. An example of the materialincluding Cr includes a Cr composite film. An example of the materialincluding nickel includes Cu—Ni (copper nickel). An example of thematerial including both of Cr and Ni includes Ni—Cr (nickel chromium).In other words, for the resistor 30, a Cr composite film may be used, ormaterial other than a Cr composite film such as Cu—Ni or Ni—Cr may beused.

Here, the Cr composite film is a composite film of Cr, CrN, Cr₂N, andthe like. The Cr composite film may include incidental impurities suchas chromium oxide. A portion of the material that constitutes thefunctional layer 20 may also be diffused into the Cr composite film. Inthis case, the material that constitutes the functional layer 20, andnitrogen may form a compound. For example, when the functional layer 20is formed of Ti, the Cr composite film may include Ti or TiN (titaniumnitride).

The thickness of the resistor 30 is not particularly restricted, and canbe appropriately selected for any purpose. The thickness can be, forexample, approximately between 0.05 μm and 2 μm. In particular, when thethickness of the resistor 30 is 0.1 μm or more, it is preferable interms of improvement in crystallinity (e.g., crystallinity of α-Cr) of acrystal that constitutes the resistor 30, and when the thickness of theresistor 30 is 1 μm or less, it is further preferable in terms ofreduction in cracks of a given film caused by internal stress of thefilm that constitutes the resistor 30, or reduction in warp in thesubstrate 10.

A line width of the resistor 30 is not particularly restricted, and canbe selected for any purpose. For example, the line width can beapproximately between 5 μm and 40 μm.

Particularly, from the viewpoints of rendering the resistor 30 highlyresistive, the thickness of the resistor 30 is preferably approximatelybetween 0.05 μm and 0.25 μm, and the line width of the resistor 30 ispreferably approximately between 5 μm and 20 μm.

With the resistor 30 being formed on the functional layer 20, theresistor 30 can be formed by a stable crystalline phase and thusstability of gauge characteristics (a gauge factor, a gauge factortemperature coefficient TCS, and a temperature coefficient of resistanceTCR) can be improved.

For example, when the resistor 30 is the Cr composite film, in a case ofproviding the functional layer 20, the resistor 30 can be formed withα-Cr (alpha-chromium) as the main component. Because α-Cr has a stablecrystalline phase, the stability of the gauge characteristics can beimproved.

Here, a main component means that a target substance is 50% by weight ormore of total substances that constitute the resistor. When the resistor30 is the Cr composite film, the resistor 30 preferably includes α-Cr at80% by weight or more, from the viewpoint of improving the gaugecharacteristics. Note that α-Cr is Cr having a bcc structure(body-centered cubic structure).

Also, by diffusing a metal (e.g., Ti) that constitutes the functionallayer 20 into the Cr composite film, the gauge characteristics can beimproved. Specifically, the gauge factor of the strain gauge 1 can be 10or more, as well as the gauge factor temperature coefficient TCS andtemperature coefficient of resistance TCR being able to be each in therange of from −1000 ppm/° C. to +1000 ppm/° C.

The resistor 30 can be formed such that L₁×L₂ is set to be in the rangeof 3 mm×3 mm. In this case, when a Cr composite film is used for theresistor 30, the relationship among the film thickness T, the line widthW, and the resistance value of the resistor 30 is given as shown inTable 1, for example. Note that in this description, as an example,spacing between lines that are next to each other is the same as theline width.

TABLE 1 LINE WIDTH [μm] 40 20 10 5 FILM RESISTANCE 2,900 11,800 49,100198,900 THICK- VALUE [Ω] NESS 0.5 μm FILM RESISTANCE 14,500 59,000245,500 994,500 THICK- VALUE [Ω] NESS 0.1 μm FILM RESISTANCE 29,000118,000 491,000 1,989,000 THICK- VALUE [Ω] NESS 0.05 μm

As shown in Table 1, when L₁×L₂ is set as 3 mm×3 mm and the filmthickness T is 0.5 μm, if the line width W is 40 μm, the resistancevalue is 2.9 kΩ. If the line width W is 20 μm, the resistance value is11. 8 kΩ. If the line width W is 10 μm, the resistance value is 49. 1kΩ. If the line width W is 5 μm, the resistance value is 198. 9 kΩ.

When L₁×L₂ is set as 3 mm×3 mm and the film thickness T is 0.1 μm, ifthe line width W is 40 μm, the resistance value is 14.5 kΩ. If the linewidth W is 20 μm, the resistance value is 59. 0 kΩ. If the line width Wis 10 μm, the resistance value is 245. 5 kΩ. If the line width W is 5μm, the resistance value is 994. 5 kΩ.

When L₁×L₂ is set as 3 mm×3 mm and the film thickness T is 0.05 μm, ifthe line width W is 40 μm, the resistance value is 29.0Ω. If the linewidth W is 20 μm, the resistance value is 118.0 kΩ. If the line width Wis 10 μm, the resistance value is 491.0 kΩ. If the line width W is 5 μm,the resistance value is 1989.0 kΩ.

The resistor 30 can be formed such that, for example, L₁×L₂ is set to bein the range of 0.3 mm×0.3 mm. In this case, when a Cr composite film isused for the resistor 30, the relationship among the film thickness T,the line width W, and the resistance value of the resistor 30 is givenas shown in Table 2, for example. Note that in this description, as anexample, spacing between lines that are next to each other is the sameas the line width.

TABLE 2 LINE WIDTH [μm] 40 20 10 5 FILM RESISTANCE 20 100 410 1,880THICK- VALUE [Ω] NESS 0.5 μm FILM RESISTANCE 100 500 2,100 9,400 THICK-VALUE [Ω] NESS 0.1 μm FILM RESISTANCE 200 1,000 4,200 18,800 THICK-VALUE [Ω] NESS 0.05 μm

As shown in Table 2, when L₁×L₂ is set as 0.3 mm×0.3 mm and the filmthickness T is 0.5 μm, if the line width W is 40 μm, the resistancevalue is 20Ω. If the line width W is 20 μm, the resistance value is100Ω. If the line width W is 10 μm, the resistance value is 410Ω. If theline width W is 5 μm, the resistance value is 1. 88 kΩ.

When L₁×L₂ is set as 0.3 mm×0.3 mm and the film thickness T is 0.1 μm,if the line width W is 40 μm, the resistance value is 100Ω. If the linewidth W is 20 μm, the resistance value is 500Ω. If the line width W is10 μm, the resistance value is 2. 1 kΩ. If the line width W is 5 μm, theresistance value is 9. 4 kΩ.

When L₁×L₂ is set as 0.3 mm×0.3 mm and the film thickness T is 0.05 μm,if the line width W is 40 μm, the resistance value is 200Ω. If the linewidth W is 20 μm, the resistance value is 1.0 kΩ. If the line width W is10 μm, the resistance value is 4.2 kΩ. If the line width W is 5 μm, theresistance value is 18. 8 kΩ.

When given resistors from among the resistors in Table 1 and Table 2constitute a Wheatstone bridge circuit, power consumption at a voltageof 1 V being applied is given as shown in Table 3 and Table 4. As shownin Table 3 and Table 4, when the resistor 30 has the decreased filmthickness T and the decreased line width W to be highly resistive, powerconsumption for the resistor 30 can be reduced.

TABLE 3 RESIS- 2,900 11,800 14,500 29,000 49,100 59,000 TANCE VALUE [Ω]POWER 344.8 84.7 69.0 34.5 20.4 16.9 CON- SUMPTION [μW] RESIS- 118,000198,900 245,500 491,000 994,500 1,989,000 TANCE VALUE [Ω] POWER 8.5 5.04.1 2.0 1.0 0.5 CON- SUMPTION [μW]

TABLE 4 RESISTANCE 20 100 200 410 500 1,000 VALUE [Ω] POWER 50,000.010,000.0 5,000.0 2,439.0 2,000.0 1,000.0 CONSUMP- TION [μW] RESISTANCE1,880 2,100 4,200 9,400 18,800 VALUE [Ω] POWER 531.9 476.2 238.1 106.453.2 CONSUMP- TION [μW]

Note that in the above description, the case where L₁×L₂ is set as 3mm×3 mm or L₁×L₂ is set as 0.3 mm×0.3 mm is an example, and L₁×L₂ is notlimited to the size described above. Further, L₁ and L₂ may be set todifferent values.

As described above, with a Cr composite film as the resistor 30 beingformed above the flexible substrate 10, the resistor 30 has thedecreased film thickness T and decreased line width W to thereby be ableto be highly resistive. When the resistor 30 is highly resistive, powerconsumption for the resistor 30 can be reduced. Further, when the linewidth W of the resistor 30 decreases, the strain gauge 1 can becomereduced in size.

Note that the expansion coefficient of the substrate 10 is preferablybetween 7 ppm/K and 20 ppm/K, from the viewpoint of reducing warp in thesubstrate 10, where the internal stress of the resistor 30 is assumed tobe close to zero. The expansion coefficient of the substrate 10 can beadjusted by, for example, selecting the material of the substrate 10,selecting the material of the filler contained in the substrate 10,adjusting the content, and the like.

When the resistor 30 is formed above the substrate 10, pinholes may begenerated in the resistor 30. If the number of pinholes generated in theresistor 30 exceeds a predetermined value, the gauge characteristicsmight deteriorate, or the resistor might not serve as a strain gauge.The inventors have recognized that one of causes of a pinhole beinggenerated in the resistor 30 relates to filler protruding from the uppersurface 10 a of the substrate 10.

In other words, when the substrate 10 includes a filler, a portion ofthe filler protrudes from the upper surface 10 a of the substrate 10, sothat surface unevenness on the upper surface 10 a of the substrate 10increases. As a result, the number of pinholes that are generated in theresistor 30 formed above the upper surface 10 a of the substrate 10increases, which results in deterioration of the gauge characteristics,and the like.

The inventors have found that, when the thickness of the resistor 30 is0.05 μm or more, in a case where the surface unevenness on the uppersurface 10 a of the substrate 10 is 15 nm or less, the number ofpinholes that are generated in the resistor 30 can be suppressed tomaintain the gauge characteristics.

In other words, when the thickness of the resistor 30 is 0.05 μm ormore, the surface unevenness on the upper surface 10 a of the substrate10 is preferably 15 nm or less, from the viewpoint of reducing thenumber of pinholes that are generated in the resistor 30 formed abovethe upper surface 10 a of the substrate 10 to maintain the gaugecharacteristics. When the surface unevenness is 15 nm or less, even in acase where the substrate 10 includes fillers, the gauge characteristicsdo not deteriorate. Note that the surface unevenness on the uppersurface 10 a of the substrate 10 may be 0 nm.

The surface unevenness on the upper surface 10 a of the substrate 10 canbe reduced by, for example, heating the substrate 10. Alternatively,instead of heating of the substrate 10, a method of scraping aprotrusion by approximately vertically irradiating the upper surface 10a of the substrate 10 with laser light; a method of cutting a protrusionby moving a water cutter or the like to be parallel to the upper surface10 a of the substrate 10; a method of polishing the upper surface 10 aof the substrate 10 with a grinding wheel; a method of pressing thesubstrate 10 while heating (heat press); or the like, may be used.

Note that the surface unevenness means arithmetical mean roughness, andis generally expressed by Ra. The surface unevenness can be measured by,for example, three-dimensional optical interferometry.

The terminal sections 41 respectively extend from both end portions ofthe resistor 30 and are each wider than the resistor 30 to be in anapproximately rectangular shape, in a plan view. The terminal sections41 are a pair of electrodes from which a change in a resistance value ofthe resistor 30 according to strain is output externally, where, forexample, a lead wire for an external connection, or the like is joined.For example, the resistor 30 extends from one of the terminal sections41, with zigzagged hairpin turns, to be connected to another terminalsection 41. The upper surface of each terminal section 41 may be coatedwith a metal allowing for increasing solderability than the terminalsection 41. Note that for the sake of convenience, the resistor 30 andthe terminal sections 41 are expressed by different numerals. However,the resistor and the terminal sections can be integrally formed of thesame material, in the same process.

The cover layer 60 is an insulating resin layer, which is disposed onand above the upper surface 10 a of the substrate 10, such that theresistor 30 is coated and the terminal sections 41 are exposed. With thecover layer 60 being provided, mechanical damage, and the like can beprevented from occurring in the resistor 30. Additionally, with thecover layer 60 being provided, the resistor 30 can be protected againstmoisture, and the like. Note that the cover layer 60 may be provided tocover all portions except for the terminal sections 41.

The cover layer 60 can be formed of an insulating resin such as a PIresin, an epoxy resin, a PEEK resin, a PEN resin, a PET resin, or a PPSresin, a composite resin (e.g., a silicone resin or a polyolefin resin).The cover layer 60 may contain fillers or pigments. The thickness of thecover layer 60 is not particularly restricted, and can be appropriatelyselected for any purpose. For example, the thickness may beapproximately between 2 μm and 30 μm.

FIGS. 3A to 3C are diagrams illustrating a process of manufacturing thestrain gauge according to the first embodiment, and each illustrate across section corresponding to FIG. 2 . In order to manufacture thestrain gauge 1, first, in the process illustrated in FIG. 3A, thesubstrate 10 is prepared and the functional layer 20 is formed on theupper surface 10 a of the substrate 10. The material and thickness foreach of the substrate 10 and the functional layer 20 are the same as thematerial and thickness described above.

The functional layer 20 can be vacuum-deposited by, for example,conventional sputtering in which a raw material capable of forming thefunctional layer 20 is the target and in which an Ar (argon) gas issupplied to a chamber. By using conventional sputtering, the functionallayer 20 is deposited while the upper surface 10 a of the substrate 10is etched with Ar. Thus, a deposited amount of film of the functionallayer 20 is minimized and thus an effect of improving adhesion can beobtained.

However, this is an example of a method of depositing the functionallayer 20, and the functional layer 20 may be formed by other methods.For example, before depositing the functional layer 20, the uppersurface 10 a of the substrate 10 is activated by plasma treatment usingAr, etc. or the like to thereby obtain the effect of improving theadhesion; subsequently, the functional layer 20 may be vacuum-depositedby magnetron sputtering.

Next, in the process illustrated in FIG. 3B, the resistor 30 and theterminal sections 41 are formed on the entire upper surface of thefunctional layer 20, and then the functional layer 20, the resistor 30,and the terminal sections 41 are each patterned in the planar shape asillustrated in FIG. 1 , by photolithography. The material and thicknessfor each of the resistor 30 and the terminal sections 41 are the same asthe material and thickness described above. The resistor 30 and theterminal sections 41 can be integrally formed of the same material. Theresistor 30 and the terminal sections 41 can be deposited by, forexample, magnetron sputtering in which a raw material capable of formingthe resistor 30 and the terminal sections 41 is a target. Instead of themagnetron sputtering, the resistor 30 and the terminal sections 41 maybe deposited by reactive sputtering, vapor deposition, arc ion plating,pulsed laser deposition, or the like.

A combination of the material of the functional layer 20 and thematerial of the resistor 30 and the terminal sections 41 is notparticularly restricted, and can be appropriately selected for anypurpose. For example, Ti is used for the functional layer 20, and a Crcomposite film formed with α-Cr (alpha-chromium) as the main componentcan be deposited as the resistor 30 and the terminal sections 41.

In this case, each of the resistor 30 and the terminal sections 41 canbe deposited by, for example, magnetron sputtering in which a rawmaterial capable of forming the Cr composite film is the target and inwhich an Ar gas is supplied to a chamber. Alternatively, the resistor 30and the terminal sections 41 may be deposited by reactive sputtering inwhich pure Cr is the target and in which an appropriate amount ofnitrogen gas, as well as an Ar gas, are supplied to a chamber.

In such methods, a growth face of the Cr composite film is defined bythe functional layer 20 formed of Ti, and a Cr composite film that isformed with α-Cr as the main component having a stable crystallinestructure can be deposited. Also, Ti that constitutes the functionallayer 20 is diffused into the Cr composite film, so that the gaugecharacteristics are improved. For example, the gauge factor of thestrain gauge 1 can be 10 or more, as well as the gauge factortemperature coefficient TCS and temperature coefficient of resistanceTCR being able to be each in the range of from −1000 ppm/° C. to +1000ppm/° C.

Note that when the resistor 30 is a Cr composite film, the functionallayer 20 formed of Ti includes all functions being a function ofpromoting crystal growth of the resistor 30; a function of preventingoxidation of the resistor 30 caused by oxygen or moisture contained inthe substrate 10; and a function of improving adhesion between thesubstrate 10 and the resistor 30. Instead of Ti, when the functionallayer 20 is formed of Ta, Si, Al, or Fe, the functional layer alsoincludes the same functions.

Next, in the process illustrated in FIG. 3C, the cover layer 60 isformed on and above the upper surface 10 a of the substrate 10, suchthat the resistor 30 is coated and the terminal sections 41 are exposed.The material and thickness of the cover layer 60 are the same as thematerial and thickness described above. For example, the cover layer 60can be fabricated, such that a thermosetting insulating resin film in asemi-cured state is laminated on the upper surface 10 a of the substrate10, and such that the resistor 30 is coated and the terminal sections 41are exposed; subsequently, heat is added and curing is performed. Thecover layer 60 may be formed, such that a thermosetting insulating resinthat is liquid or paste-like is applied to the upper surface 10 a of thesubstrate 10, and such that the resistor 30 is coated and the terminalsections 41 are exposed; subsequently, heat is added and curing isperformed. In the above process, the strain gauge 1 is completed.

As described above, with the functional layer 20 being provided in thelower layer of the resistor 30, the crystal growth of the resistor 30can be promoted and thus the resistor 30 having a stable crystallinephase can be fabricated. As a result, with respect to the strain gauge1, the stability of the gauge characteristics can be improved. Also, thematerial that constitutes the functional layer 20 is diffused into theresistor 30, so that the gauge characteristics of the strain gauge 1 canbe thereby improved.

<Modification 1 of the First Embodiment>

Modification 1 of the first embodiment provides an example of a straingauge in which an insulating layer is provided in a lower layer of thecover layer. Note that in the modification 1 of the first embodiment,the explanation for the same components as the embodiment that has beendescribed may be omitted.

FIG. 4 is a cross-sectional view illustrating an example of the straingauge according to the modification 1 of the first embodiment, andillustrates a cross section corresponding to FIG. 2 . With reference toFIG. 4 , the strain gauge 1A differs from the strain gauge 1 (see FIGS.1 and 2 , etc.) in that an insulating layer 50 is provided in the lowerlayer of the cover layer 60. Note that the cover layer 60 may beprovided to cover all portions except for the terminal sections 41.

The insulating layer 50 is provided on and above the upper surface 10 aof the substrate 10, such that the resistor 30 is coated and theterminal sections 41 are exposed. For example, the cover layer 60 can beprovided to cover a portion of a side surface of the insulating layer50, and an upper surface thereof.

The material of the insulating layer 50 is not particularly restrictedas long as the material has higher resistance than the resistor 30 andthe cover layer 60. The material can be appropriately selected for anypurpose. For example, an oxide or a nitride, such as Si, W, Ti, or Ta,can be used. The thickness of the insulating layer 50 is notparticularly restricted, and can be appropriately selected for anypurpose. For example, the thickness can be approximately between 0.05 μmand 1 μm.

The method of forming the insulating layer 50 is not particularlyrestricted, and can be appropriately selected for any purpose. Forexample, a vacuum process such as sputtering or chemical vapordeposition (CVD), or, a solution process such as spin coating or asol-gel process can be used.

In such a manner, with the insulating layer 50 being provided in thelower layer of the cover layer 60, insulation and environmental sealingcan be improved in comparison to the case where the cover layer 60 aloneis used. In such a manner, the insulating layer 50 can be appropriatelyprovided according to a specification required for the insulation andenvironmental sealing.

Second Embodiment

The second embodiment provides an example of a strain gauge in whicheach electrode has a laminated structure. Note that in the secondembodiment, the explanation for the same components as the embodimentthat has been described may be omitted.

FIG. 5 is a plan view illustrating an example of a strain gaugeaccording to the second embodiment. FIG. 6 is a cross-sectional viewillustrating an example of the strain gauge according to the secondembodiment, and illustrates a cross section taken along the line B-B inFIG. 5 . With reference to FIGS. 5 and 6 , the strain gauge 2 includeselectrodes 40A in each of which a plurality of layers are laminated.Note that the cover layer 60 may be provided to cover all portionsexcept for the electrodes 40A.

Each electrode 40A has a laminated structure in which a plurality ofmetallic layers are laminated. Specifically, each electrode 40A includesa terminal section 41 extending from a corresponding end portion fromamong both end portions of the resistor 30; a metallic layer 42 formedon an upper surface of the terminal section 41; a metallic layer 43formed on an upper surface of the metallic layer 42; and a metalliclayer 44 formed on an upper surface of the metallic layer 43. Themetallic layer 43 is a typical example of a first metallic layeraccording to the present invention, and the metallic layer 44 is atypical example of a second metallic layer according to the presentinvention.

The material of the metallic layer 42 is not particularly restricted,and can be appropriately selected for any purpose. For example, Cu(copper) can be used. The thickness of the metallic layer 42 is notparticularly restricted, and can be appropriately selected for anypurpose. For example, the thickness can be approximately in the range offrom 0.01 μm to 1 μm.

Preferably, the material of the metallic layer 43 includes Cu, a Cualloy, Ni, or a Ni alloy. The thickness of the metallic layer 43 isdetermined in consideration of solderability to the electrode 40A, andis preferably 1 μm or more, and more preferably 3 μm or more. When thematerial of the metallic layer 43 includes Cu, a Cu alloy, Ni, or a Nialloy and the thickness of the metallic layer 43 is 1 μm or more,dissolution of metallization is ameliorated. Also, when the material ofthe metallic layer 43 includes Cu, a Cu alloy, Ni, or a Ni alloy and thethickness of the metallic layer 43 is 3 μm or more, dissolution ofmetallization is further ameliorated. Note that the thickness of themetallic layer 43 is preferably 30 μm or less in terms of ease ofelectrolytic plating.

Here, the dissolution of metallization means that the materialconstituting the electrode 40A is dissolved in solder for jointing theelectrode 40A, and that the thickness of the electrode 40A is reduced orthe material disappears. When the dissolution of metallization occurs,adhesion strength or tensile strength with a lead wire, or the like tobe jointed to the electrode 40A may be reduced. Thus, it is preferablethat no dissolution of metallization occur.

For the material of the metallic layer 44, material having increasingsolder wettability than the metallic layer 43 can be selected. Forexample, when the material of the metallic layer 43 includes Cu, a Cualloy, Ni, or a Ni alloy, the material of the metallic layer 44 caninclude Au (gold). When the surface of Cu, a Cu alloy, Ni, or a Ni alloyis coated with Au, oxidation and corrosion for Cu, a Cu alloy, Ni, or aNi alloy can be prevented, as well as increased solder wettability canbe provided. Instead of Au, when the material of the metallic layer 44includes Pt (platinum), the metallic layer 44 has the same effect. Thethickness of the metallic layer 44 is not particularly restricted, andcan be appropriately selected for any purpose. For example, thethickness can be approximately between 0.01 μm and 1 μm.

Note that each terminal section 41 is exposed around a given laminatedsection of the metallic layers 42, 43, and 44, in a plan view. However,each terminal section 41 may have the same planar shape as the laminatedsection of the metallic layers 42, 43, and 44.

FIGS. 7A to 8D illustrate a process of manufacturing a strain gaugeaccording to a second embodiment, and illustrate a cross sectioncorresponding to FIG. 6 . In order to manufacture the strain gauge 2, aprocess that is similar to that in FIG. 3A according to the firstembodiment is first performed, and then in the process illustrated inFIG. 7A, a metallic layer 300 is formed on an upper surface of thefunctional layer 20. The metallic layer 300 is a layer that is finallypatterned to serve as the resistor 30 and terminal sections 41. In sucha manner, the material and thickness of the metallic layer 300 are thesame as the material and thickness for each of the above resistor 30 andterminal sections 41.

The metallic layer 300 can be deposited by magnetron sputtering inwhich, for example, a raw material capable of forming the metallic layer300 is the target. Instead of the magnetron sputtering, the metalliclayer 300 may be deposited by reactive sputtering, vapor deposition, arcion plating, pulsed laser deposition, or the like.

Next, in the process illustrated in FIG. 7B, a seed layer 420 as themetallic layer 42 is formed by, for example, sputtering, electrolessplating, or the like, to cover an upper surface of the metallic layer300.

Next, in the process illustrated in FIG. 7C, a photosensitive resist 800is formed on the entire upper surface of the seed layer 420, and byexposing and developing, an opening 800 x for exposing a region in whicheach electrode 40A is to be formed is formed. As the resist 800, forexample, a dry film resist, or the like can be used.

Next, in the process illustrated in FIG. 7D, a given metallic layer 43is formed on the seed layer 420 that is exposed in the opening 800 x, byfor example, electrolytic plating in which the seed layer 420 is set asa power supply path, and further, a given metallic layer 44 is formed onthe metallic layer 43. The electrolytic plating is suitable because ithas high takt and allows for formation of a low stress electrolyticplating layer as the metallic layer 43. When the electrolytic platinglayer whose thickness is increased has low stress, warp in the straingauge 2 can be prevented.

Note that in forming the metallic layer 44, side surfaces of themetallic layer 43 are coated with the resist 800, so that the metalliclayer 44 is formed only on the upper surface of the metallic layer 43and is not on the side surfaces thereof.

Next, in the process illustrated in FIG. 8A, the resist 800 illustratedin FIG. 7D is removed. The resist 800 can be removed by, for example,immersing the material of the resist 800 in a dissolvable solution.

Next, in the process illustrated in FIG. 8B, a photosensitive resist 810is formed on the entire upper surface of the seed layer 420, and byexposing and developing, a planar shape that is the same as that of theresistor 30 and terminal sections 41 in FIG. 5 is patterned. As theresist 810, for example, a dry film resist, or the like can be used.

Next, in the process illustrated in FIG. 8C, the resist 810 is used asan etch mask, and the functional layer 20, the metallic layer 300, andthe seed layer 420 that are exposed from the resist 810 are removed, sothat the functional layer 20, the resistor 30, and the terminal sections41 each of which has the planar shape in FIG. 5 are formed. For example,with wet etching, unwanted portions of the functional layer 20; themetallic layer 300; and the seed layer 420 can be removed. Note that atthis point, the seed layer 420 is formed on the resistor 30.

Next, in the process illustrated in FIG. 8D, the metallic layer 43 andthe metallic layer 44 are used as etch masks, and an unwanted seed layer420 that is exposed from the metallic layer 43 and the metallic layer 44is removed, so that the metallic layer 42 is formed. For example, by wetetching using etching liquid with which the seed layer 420 is etched andwith which the functional layer 20 and the resistor 30 are not etched,the unwanted seed layer 420 can be removed.

After the process illustrated in FIG. 8D, as is the case with theprocess in FIG. 3C, the cover layer 60 with which the resistor 30 iscoated and that exposes the electrodes 40A is formed on and above theupper surface 10 a of the substrate 10, so that the strain gauge 2 iscompleted.

As described above, as each electrode 40A, a given metallic layer 43formed of a thick film (1 μm or more), which is formed of Cu, a Cualloy, Ni, or a Ni alloy, is formed above a given terminal section 41,and further, a given metallic layer 44 formed of material (Au or Pt)that has increasing solder wettability than the metallic layer 43 isformed in the outermost surface layer. Thereby, dissolution ofmetallization can be prevented, as well as improving solder wettability.

<Modification 1 of the Second Embodiment>

Modification 1 of the second embodiment provides an example ofelectrodes each having a layer structure different from that in thesecond embodiment. Note that in the modification 1 of the secondembodiment, the explanation for the same components as the embodimentsthat have been described may be omitted.

FIG. 9 is a cross-sectional view illustrating an example of a straingauge according to the modification 1 of the second embodiment, andillustrates a cross section corresponding to FIG. 6 . With reference toFIG. 9 , the strain gauge 2A differs from the strain gauge 2 (see FIG. 6, etc.) in that the electrodes 40A are replaced with electrodes 40B.Additionally, the cover layer 60 is provided to approximately cover allportions except for the electrodes 40B, which differs from the straingauge 2 (see FIG. 6 , etc.).

Each electrode 40B has a laminated structure in which a plurality ofmetallic layers are laminated. Specifically, each electrode 40B includesa terminal section 41 extending from a corresponding end portion fromamong both end portions of the resistor 30; a metallic layer 42 formedon an upper surface of the terminal section 41; a metallic layer 43formed on an upper surface of the metallic layer 42; a metallic layer 45formed on an upper surface of the metallic layer 43; and a metalliclayer 44 formed on an upper surface of the metallic layer 45. In otherwords, each electrode 40B has a structure in which the metallic layer 45is provided between the metallic layer 43 and the metallic layer 44 of agiven electrode 40A.

The material of the metallic layer 45 is not particularly restricted,and can be appropriately selected for any purpose. For example, Ni canbe used. Instead of Ni, NiP (nickel phosphorus) or Pd may be used. Also,as the metallic layer 45, Ni/Pd (a metallic layer in which a Ni layerand a Pd layer are laminated in this order) may be used. The thicknessof the metallic layer 45 is not particularly restricted, and can beappropriately selected for any purpose. For example, the thickness isapproximately between 1 μm and 2 μm.

In the process illustrated in FIG. 7D, the metallic layer 45 can beformed on the metallic layer 43 by, for example, electrolytic plating inwhich the seed layer 420 is set as a power supply path.

In such a manner, the number of electrode layers is not particularlyrestricted, and the number of layers may be increased as necessary. Inthis case as well, a given metallic layer 43 formed of a thick film (1μm or more), which is formed of Cu, a Cu alloy, Ni, or a Ni alloy, isformed above a given terminal section 41, and further, a given metalliclayer 44 formed of material (Au or Pt) that has increasing solderwettability than the metallic layer 43 is formed in the outermostsurface layer. Thereby, as is the case with the second embodiment, thedissolution of metallization can be prevented, as well as improving thesolder wettability.

<Modification 2 of the Second Embodiment>

Modification 2 of the second embodiment provides another example ofelectrodes each having a different layer structure from that in thesecond embodiment. Note that in the modification 2 of the secondembodiment, the explanation for the same components as the embodimentsthat have been described may be omitted.

FIG. 10 is a cross-sectional view illustrating an example of a straingauge according to the modification 2 of the second embodiment, andillustrates a cross section corresponding to FIG. 6 . With reference toFIG. 10 , the strain gauge 2B differs from the strain gauge 2A (see FIG.9 ) in that the electrodes 40B are replaced with electrodes 40C.Additionally, the cover layer 60 is provided to approximately cover allportions except for the electrodes 40C, which differs from the straingauge 2 (see FIG. 6 , etc.).

Each electrode 40C has a laminated structure in which a plurality ofmetallic layers are laminated. Specifically, each electrode 40C includesa terminal section 41 extending from a corresponding end portion fromamong both end portions of the resistor 30; a metallic layer 42 formedon an upper surface of the terminal section 41; a metallic layer 43formed on an upper surface of the metallic layer 42; a metallic layer45A formed on an upper surface and side surfaces of the metallic layer43 and on side surfaces of the metallic layer 42; and a metallic layer44A formed on an upper surface and side surfaces of the metallic layer45A. For example, the material and thickness for each of the metalliclayers 44A and 45A can be the same as the material and thickness of themetallic layers 44 and 45. Note that the metallic layer 44A is a typicalexample of a second metallic layer according to the present invention.

In order to form each electrode 40C, first, in the process illustratedin FIG. 7D, for example, a given metallic layer 43 is formed by, forexample, electrolytic plating in which the seed layer 420 is set as apower supply path, and then the resist 800 is removed as is the casewith the process illustrated in FIG. 8A, without forming a givenmetallic layer 44. Next, the same process as that in FIGS. 8B to 8D isperformed. Subsequently, a given metallic layer 45A can be formed on theupper surface and side surfaces of the metallic layer 43 and on the sidesurfaces of the metallic layer 42, by electroless plating, for example.Additionally, a given metallic layer 44A can be formed on the uppersurface and side surfaces of the metallic layer 45A, by electrolessplating, for example.

As described above, each electrode can be fabricated by appropriatelyusing both of electrolytic plating and electroless plating. In thestructure of each electrode 40C, a given metallic layer 43 formed of athick film (1 μm or more), which is formed of Cu, a Cu alloy, Ni, or aNi alloy, is formed above a given terminal section 41, and further, agiven metallic layer 44A formed of material (Au or Pt) that hasincreasing solder wettability than the metallic layer 43 is formed inthe outermost layer. Note, however, that the metallic layer 44A of theoutermost layer is formed, via the metallic layer 45A, toward the sidesurfaces of each of the metallic layers 42 and 43, in addition to theupper surface of the metallic layer 43. Thus, in comparison to theelectrodes 40A or the electrodes 40B, the effect of preventing oxidationand corrosion of Cu, a Cu alloy, Ni, or a Ni alloy that constitutes themetallic layer 43 can be further enhanced, as well as the solderwettability can be further improved.

Note that the same effect is obtained even when a given metallic layer44A is formed directly on the upper surface and side surfaces of a givenmetallic layer 43 and on the side surfaces of a given metallic layer 42,without forming a given metallic layer 45A. In other words, the metalliclayer 44A may directly or indirectly cover the upper surface and sidesurfaces of the metallic layer 43 and the side surfaces of the metalliclayer 42.

Third Embodiment

A third embodiment provides an example of a sensor module using a straingauge. Note that in the third embodiment, the explanation for the samecomponents as the embodiments that have been described may be omitted.

FIG. 11 is a cross-sectional view illustrating an example of the sensormodule according to the third embodiment, and illustrates a crosssection corresponding to FIG. 2 . With reference to FIG. 11 , the sensormodule 5 includes the strain gauge 1, a flexure element 110, and anadhesive layer 120. Note that the cover layer 60 may be provided tocover all portions except for the terminal sections 41.

In the sensor module 5, an upper surface 110 a of the flexure element110 is secured to the lower surface 10 b of the substrate 10, via theadhesive layer 120. For example, the flexure element 110 is an objectthat is formed of a metal such as Fe, SUS (stainless steel), or Al, or,a resin such as PEEK, and that is deformed (causes strain) according toforce that is applied. The strain gauge 1 can detect strain generated inthe flexure element 110, as a change in a resistance value of theresistor 30.

The material of the adhesive layer 120 is not particularly restricted aslong as it has a function of securing the flexure element 110 to thestrain gauge 1. The material can be appropriately selected for anypurpose. For example, an epoxy resin, a modified epoxy resin, a siliconeresin, a modified silicone resin, a urethane resin, a modified urethaneresin, or the like can be used. Also, material such as a bonding sheetmay be used. The thickness of the adhesive layer 120 is not particularlyrestricted, and can be appropriately selected for any purpose. Forexample, the thickness can be approximately between 0.1 μm and 50 μm.

In order to manufacture the sensor module 5, after the strain gauge 1 isfabricated, for example, any material described above, which constitutesthe adhesive layer 120, is applied to the lower surface 10 b of thesubstrate 10 and/or the upper surface 110 a of the flexure element 110.Then, the lower surface 10 b of the substrate 10 is situated facing theupper surface 110 a of the flexure element 110, and the strain gauge 1is disposed above the flexure element 110, through the applied material.Alternatively, the bonding sheet may be interposed between the flexureelement 110 and the substrate 10.

Next, the strain gauge 1 is heated to a predetermined temperature whilebeing pressed toward the flexure element 110, and the applied materialis cured, so that the adhesive layer 120 is formed. Thereby, the lowersurface 10 b of the substrate 10 is secured to the upper surface 110 aof the flexure element 110, through the adhesive layer 120, so that thesensor module 5 is completed. For example, the sensor module 5 can beapplied in measurement of load, pressure, torque, acceleration, or thelike.

Note that for the sensor module 5, the strain gauge 1A, 2, 2A, or 2B maybe used instead of the strain gauge 1.

Example 1

First, in an advance test, Ti as the functional layer 20 wasvacuum-deposited on the upper surface 10 a of the substrate 10 formed ofa polyimide resin that had a thickness of 25 μm, by conventionalsputtering. In this case, five samples for each of which Ti wasdeposited were fabricated in order to target multiple film thicknesses.

Next, for the fabricated five samples, X-ray fluorescence (XRF) analysiswas performed to obtain the result as illustrated in FIG. 12 . From anX-ray peak in FIG. 12 , it was confirmed that Ti was present, and fromX-ray intensity of each sample at the X-ray peak, it was confirmed thata film thickness of a given Ti film could be controlled to be in therange of from 1 nm to 100 nm.

Next, in Example 1, Ti as the functional layer 20, which had a filmthickness of 3 nm, was vacuum-deposited on the upper surface 10 a of thesubstrate 10 formed of a polyimide resin that had a thickness of 25 μm,by conventional sputtering.

Subsequently, a Cr composite film, as the resistor 30 and the terminalsections 41, was deposited on the entire upper surface of the functionallayer 20, by magnetron sputtering, and then the functional layer 20, theresistor 30, and the terminal sections 41 were patterned byphotolithography, as illustrated in FIG. 1 .

In comparative example 1, without forming the functional layer 20, a Crcomposite film, as the resistor 30 and the terminal sections 41, wasdeposited on the upper surface 10 a of the substrate 10 formed of apolyimide resin that had a thickness of 25 μm, by magnetron sputtering.Then, patterning was performed by photolithography, as illustrated inFIG. 1 . Note that for the sample used in Example 1 and the sample usedin comparative example 1, all deposition conditions for the resistor 30and the terminal sections 41 are the same.

Next, for a given sample used in Example 1 and a given sample used incomparative example 1, X-ray diffraction evaluation was performed toobtain the result illustrated in FIG. 13 . FIG. 13 illustrates an X-raydiffraction pattern at a diffraction angle of 28 being in the range offrom 36 to 48 degrees, and a diffraction peak in Example 1 is shifted tothe right in comparison to a diffraction peak in comparative example 1.Further, the diffraction peak in Example 1 is greater than thediffraction peak in comparative example 1.

The diffraction peak in Example 1 is situated in proximity to adiffraction line of α-Cr (110). This is considered that when thefunctional layer 20 formed of Ti was provided, crystal growth of α-Crwas promoted to thereby form a Cr composite film with α-Cr as the maincomponent.

Next, multiple samples used in Example 1 and comparative example 1 werefabricated, and gauge characteristics were measured. As a result, agauge factor for each sample in Example 1 was between 14 and 16. Incontrast, a gauge factor for each sample in comparative example 1 wasless than 10.

Also, for each sample in Example 1, the gauge factor temperaturecoefficient TCS and temperature coefficient of resistance TCR were eachin the range of from −1000 ppm/° C. to +1000 ppm/° C. In contrast, foreach sample in comparative example 1, the gauge factor temperaturecoefficient TCS and temperature coefficient of resistance TCR were noteach in the range of from −1000 ppm/° C. to +1000 ppm/° C.

As described above, with the functional layer 20 formed of Ti beingprovided, crystal growth of α-Cr was promoted and a Cr composite filmwas formed with α-Cr as the main component, so that a strain gauge thathad a gauge factor of 10 or more, and that had the gauge factortemperature coefficient TCS and temperature coefficient of resistanceTCR being each in the range of from −1000 ppm/° C. to +1000 ppm/° C.,was fabricated. Note that the diffusion effect of Ti into the Crcomposite film is considered to cause the improvement in the gaugecharacteristics.

Example 2

In Example 2, multiple substrates 10 each formed of a polyimide resinthat had a thickness of 25 μm and that had a different expansioncoefficient were prepared. Then, when a Cr-composite film, as a givenresistor 30, was deposited, a relationship between an expansioncoefficient of a given substrate 10 and internal stress of the resistor30 was checked, to thereby obtain the result illustrated in FIG. 14 .

The internal stress of the resistor 30 was estimated by measuring warpin an evaluation sample and using the Stoney formula given by Formula(1). Note that as can be seen from Formula (1), the internal stress ofthe resistor 30 illustrated in FIG. 14 indicates a value per unitthickness and does not depend on the thickness of the resistor 30.[Math. 1]INTERNAL STRESS=ED2/6(1−v)tR  (1)

Note that in Formula (1), E denotes the Young's modulus, v denotes aPoisson's ratio, D denotes the thickness of the substrate 10, t denotesthe thickness of the resistor 30, and R denotes change in radius ofcurvature in the substrate 10.

From FIG. 14 , when the expansion coefficient of the substrate 10 is inthe range of from 7 ppm/K to 20 ppm/K, the internal stress of theresistor 30 can be maintained to be in the range of ±0.4 GPa. Where,±0.4 GPa indicates values expressing a permittable warp in the straingauge 1 for functioning, and was experimentally determined by theinventors.

In other words, when the expansion coefficient of the substrate 10 isout of the range of from 7 ppm/K to 20 ppm/K, the internal stress of theresistor 30 is out of the range of ±0.4 GPa and thus warp in the straingauge 1 would increase, so that the strain gauge 1 would not function asa strain gauge. Therefore, the expansion coefficient of the substrate 10is required to be in the range of from 7 ppm/K to 20 ppm/K. Note thatthe material of the substrate 10 does not necessarily include apolyimide resin.

The expansion coefficient of the substrate 10 can be in the range offrom 7 ppm/K to 20 ppm/K, by selecting the material of the substrate 10,selecting the material of the filler contained in the substrate 10,adjusting the content, and the like.

As described above, with the expansion coefficient of the substrate 10being in the range of from 7 ppm/K to 20 ppm/K, a difference in theexpansion coefficient between the substrate 10 and the resistor 30, aswell as other factors, are absorbed, so that the internal stress of theresistor 30 can be in the range of ±0.4 GPa. As a result, warp in thestrain gauge 1 is reduced to thereby cause the strain gauge 1 to be ableto function stably in a manner such that good gauge characteristics aremaintained.

Example 3

In Example 3, multiple substrates 10 each formed of a polyimide resinthat had a thickness of 25 μm and that contained fillers were prepared.Three sets of samples, each of which included a sample not being subjectto heat treatment; a sample being subject to heat treatment at atemperature of 100° C.; a sample being subject to heat treatment at atemperature of 200° C.; and a sample being subject to heat treatment ata temperature of 300° C., were fabricated. Then, the samples werereturned to be at normal temperature, and surface unevenness on theupper surface 10 a of each substrate 10 was measured bythree-dimensional optical interference.

Next, the resistor 30 having a film thickness of 0.05 μm was depositedon the upper surface 10 a of each substrate 10, by magnetron sputtering,and patterning was performed by photolithography, as illustrated in FIG.1 . Then, the number of pinholes that were generated in the resistor 30was measured by a light transmission method in which light wastransmitted from a back surface of a given sample.

Next, based on a measured result, a relationship between surfaceunevenness on the upper surface 10 a of a given substrate 10 and thenumber of pinholes that were generated in a given resistor 30 wassummarized in FIG. 15 . Note that each bar graph illustrated in FIG. 15shows surface unevenness, and a line graph shows the number of pinholes.Additionally, for the horizontal axis, 100° C., 200° C., and 300° C.each indicate a temperature when a given substrate 10 was subject toheat treatment, and Incomplete indicates that heat treatment is notcarried out.

FIG. 15 indicates that when a given substrate 10 is heated attemperatures between 100° C. and 300° C., the surface unevenness on theupper surface 10 a of the substrate 10 is 15 nm or less, which is abouthalf of surface unevenness in a case of being incomplete, and that as aresult, the number of pinholes in the resistor 30 is drastically reducedto about 1/7. Note, however, that in consideration of resistance tothermal temperature of a polyimide resin, when heat treatment is carriedout at temperatures exceeding 250° C., alteration or deterioration mayoccur. Accordingly, it is preferable that the heat treatment be carriedout at temperatures between 100° C. and 250° C. Note that it isconsidered that the surface unevenness is reduced by heat treatmentbecause fillers are contained in a polyimide resin that constitutes thesubstrate 10, during thermal shrinkage caused by the heat treatment.

According to consideration by the inventors, the number of pinholes(about 140) in the case of Incomplete, as illustrated in FIG. 15 ,indicates a level of the gauge characteristics deteriorating. Incontrast, the number of pinholes (about 20) after heat treatment,indicates a level of the gauge characteristics not being adverselyaffected. In other words, when the resistor 30 having a film thicknessof 0.05 μm is used, in a case where the surface unevenness on the uppersurface 10 a of the substrate 10 is 15 nm or less, it was confirmed thatthe number of pinholes that were generated in the resistor 30 could bereduced to indicate a level of the gauge characteristics not beingadversely affected.

Note that when the resistor 30 having a film thickness of greater than0.05 μm is used, it is obvious that when the surface unevenness on theupper surface 10 a of the substrate 10 is 15 nm or less, the number ofpinholes that are generated in the resistor 30 can be reduced toindicate a level of the gauge characteristics not being adverselyaffected. In other words, with the surface unevenness on the uppersurface 10 a of the substrate 10 being 15 nm or less, when the resistor30 having a film thickness of 0.05 μm or more is used, the number ofpinholes that are generated in the resistor 30 can be reduced toindicate a level of the gauge characteristics not being adverselyaffected.

As described above, with the substrate 10 being subject to heattreatment, the surface unevenness on the upper surface 10 a of thesubstrate 10 can be 15 nm or less, and as a result, the number ofpinholes that are generated in the resistor 30 having a film thicknessof 0.05 μm or more can be significantly reduced. As a result, the straingauge 1 can function stably in a manner such that good gaugecharacteristics are maintained.

Note that in order to reduce the number of pinholes that are generatedin the resistor 30, it is important to reduce the surface unevenness onthe upper surface 10 a of the substrate 10, and a method of reducingsurface unevenness is not important. In the above description, themethod of reducing surface unevenness by heat treatment has beendescribed, but is not limited to this case. Any method may be used aslong as the surface unevenness on the upper surface 10 a of thesubstrate 10 can be reduced.

The surface unevenness on the upper surface 10 a of the substrate 10 canbe reduced by, for example, a method of scraping a protrusion byapproximately vertically irradiating the upper surface 10 a of thesubstrate 10, with laser light; a method of cutting a protrusion bymoving a water cutter or the like to be parallel to the upper surface 10a of the substrate 10; a method of polishing the upper surface 10 a ofthe substrate 10 with a grinding wheel; a method of pressing thesubstrate 10 while heating (heat press); or the like.

Further, in order to reduce the number of pinholes that are generated inthe resistor 30, it is important to reduce the surface unevenness on theupper surface 10 a of the substrate 10, and is not necessarily limitedto being directed to surface unevenness caused by the fillers that arepresent. It is effective to reduce surface unevenness not being causedby the fillers that are present, by various methods described above. Forexample, when surface unevenness on the substrate 10 without containingfillers is greater than 15 nm, in a case where the surface unevenness onthe upper surface 10 a of the substrate 10 is 15 nm or less, by variousmethods described above, the number of pinholes that are generated inthe resistor 30 having a film thickness of 0.05 μm or more can bereduced to a level of the gauge characteristics not being adverselyaffected.

Example 4

In Example 4, the process illustrated in FIGS. 7A to 8D was modified asdescribed in the modification 1 of the second embodiment, the straingauge 2A with the electrodes 40B was fabricated, and the presence orabsence of dissolution of metallization was checked. Specifically, 10types of samples in each of which Cu was used for the metallic layers 42and 43, in each of which NiP was used for the metallic layer 45, in eachof which Au was used for the metallic layer 44, and in each of which thethickness of a given metallic layer was changed were fabricated (samplesNo. 1 to No. 10), and then the presence or absence of dissolution ofmetallization was checked.

Table 5 shows results. Note that in Table 5, the film thickness “0”indicates that no metallic layer was formed. The “poor” indicates thatdissolution of metallization occurred in soldering being firstperformed. The “good” indicates that although no dissolution ofmetallization occurred in soldering being first performed, littledissolution of metallization occurred in soldering being performedsecond (where soldering refinement, etc. was assumed). Additionally, the“excellent” indicates that dissolution of metallization occurred neitherin soldering being performed first nor second.

TABLE 5 EACH FILM THICKNESS SAMPLE [μm] No. Cu NiP Au SOLDERABILITY 10.5 0 0 POOR PRESENCE OF DISSOLUTION OF METALLIZATION 2 0.5 0 0.1 POORPRESENCE OF DISSOLUTION OF METALLIZATION 3 0.5 0 0.8 POOR PRESENCE OFDISSOLUTION OF METALLIZATION 4 1 1 0.1 GOOD ABSENCE OF DISSOLUTION OFMETALLIZATION 5 3 0 0 EXCELLENT ABSENCE OF DISSOLUTION OF METALLIZATION6 3 0 0.1 EXCELLENT ABSENCE OF DISSOLUTION OF METALLIZATION 7 3 0 0.8EXCELLENT ABSENCE OF DISSOLUTION OF METALLIZATION 8 3 1 0.1 EXCELLENTABSENCE OF DISSOLUTION OF METALLIZATION 9 5 1 0.1 EXCELLENT ABSENCE OFDISSOLUTION OF METALLIZATION 10 8 1 0.1 EXCELLENT ABSENCE OF DISSOLUTIONOF METALLIZATION

As shown in Table 5, it was confirmed that when the thickness of Cu was1 μm or more, dissolution of metallization was improved, and that whenthe thickness was 3 μm or more, the dissolution of metallization wasfurther improved. Additionally, from the results for sample 1 and sample5, it was confirmed that the presence or absence of dissolution ofmetallization was determined only according to the thickness of Cu andwas not determined upon the presence or absence of each of NiP and Au.Note, however, that as described above, in order to prevent dissolutionof metallization and improve solder wettability, a metallic layer formedof Au or an equivalent material (Pt, etc.) is required.

The preferred embodiments and the like have been described above indetail, but are not limited thereto. Various modifications andalternatives to the above embodiments and the like can be made withoutdeparting from a scope set forth in the claims.

This International application claims priority to Japanese PatentApplication No. 2017-246871, filed Dec. 22, 2017, the contents of whichare incorporated herein by reference in their entirety.

Reference Signs List

1, 1A, 2, 2A, 2B strain gauge, 5 sensor module, 10 substrate, 10 a uppersurface, 20 functional layer, 30 resistor, 41 terminal section, 40A,40B, 40C electrode, 42, 43, 44, 44A, 45, 45A metallic layer, 50insulating layer, 60 cover layer, 110 flexure element, 120 adhesivelayer

The invention claimed is:
 1. A strain gauge comprising: a flexible resin substrate; a functional layer including a chemical material and formed of a metal, an alloy, or a metal compound, on one surface of the substrate; a resistor formed as a film containing Cr, CrN, and Cr₂N, on one surface of the functional layer, wherein the chemical material is diffused into the film; and an insulating resin layer with which the resistor is coated, wherein the functional layer promotes crystal growth of the resistor.
 2. The strain gauge according to claim 1, wherein a film thickness of the resistor is between 0.05 μm and 0.5 μm.
 3. The strain gauge according to claim 1, wherein a line width of the resistor is between 5 μm and 40 μm.
 4. The strain gauge according to claim 1, wherein the substrate has an expansion coefficient in a range of from 7 ppm/K to 20 ppm/K.
 5. The strain gauge according to claim 1, wherein surface unevenness on the one surface of the substrate is 15 nm or less, and wherein the resistor has a film thickness of 0.05 μm or more.
 6. The strain gauge according to claim 1, further comprising electrodes electrically coupled to the resistor, wherein each electrode includes: a terminal section extending from a given end portion of the resistor; a first metallic layer formed of copper, a copper alloy, nickel, or a nickel alloy, on or above the terminal section; and a second metallic layer formed of material having greater solder wettability than the first metallic layer, on or above the first metallic layer.
 7. The strain gauge according to claim 1, further comprising an insulating layer which is formed, in a lower layer of the insulating resin layer, of material having higher resistance than the resistor and the insulating resin layer, and with which the resistor is coated.
 8. The strain gauge according to claim 1, wherein a main component of the resistor is alpha-chromium.
 9. The strain gauge according to claim 8, wherein the resistor includes alpha-chromium at 80% by weight or more.
 10. The strain gauge according to claim 1, wherein the functional layer includes titanium.
 11. The strain gauge according to claim 10, wherein the resistor includes titanium.
 12. The strain gauge according to claim 10, wherein the resistor includes titanium nitride.
 13. A sensor module comprising: the strain gauge according to claim 1; and a flexure element disposed on another surface of the substrate. 